1. Field of the Invention
The present invention relates generally to interbody spinal implants preferably adapted for placement in pairs side by side to either side of the midline with or without a space therebetween into a space created across the height of a disc space and between two adjacent vertebral bodies, after the removal of damaged spinal disc material, for the purpose of correcting spinal disease at that interspace. The spinal implants are made of an implant material that is other than bone and may or may not be resorbable. Where the implants are spinal fusion implants, they are adapted such that fusion occurs at least in part through the implants themselves. Where the implants are motion preserving for maintaining spinal motion, bone growth can occur at least in part into the spinal implants themselves, but not across them, and they are adapted to allow for relative motion between the vertebrae.
2. Description of the Related Art
Surgical interbody spinal fusion generally refers to the methods for achieving a bridge of bone tissue in continuity between adjacent vertebral bodies and across the disc space to thereby substantially eliminate relative motion between the adjacent vertebral bodies. The term “disc space” refers to the space between adjacent vertebral bodies normally occupied by a spinal disc.
Motion preserving implants maintain the spacing between the two adjacent vertebral bodies and allow for relative motion between the vertebrae. Bone growth from the adjacent vertebral bodies into the motion preserving implant, but not through the implant, anchors the implant to the adjacent vertebral bodies while preserving the relative motion between the vertebrae.
Spinal implants can have opposed upper and lower surfaces that are arcuate or non-arcuate transverse to the longitudinal axis of the implant along at least a portion of the length of the implant. Implants having arcuate opposed portions are adapted to be implanted across and beyond the height of the restored disc space, generally into a bore formed across the height of a disc space. Some of the advantages offered by implants with arcuate opposed portions include: 1) the installation of the implant into vascular bone made possible by the creation of a bore into the bone of the adjacent vertebral bodies; 2) the implant's geometric shape is easy to manufacturer; 3) the implant can include external threads to facilitate insertion into the implantation space; and 4) the implant provides more surface area to contact the adjacent vertebral bodies than would a flat surface. Some disadvantages associated with implants having arcuate opposed portions include; 1) the creation of a bore into the adjacent vertebral bodies to form the implantation space results in a loss of the best structural bone of the vertebral endplate; 2) the implant needs to have a larger cross section to fill the prepared implantation site which may be more difficult to install, especially from a posterior approach; and 3) the width of the implant is generally related to the height of the implant, so if the implant is for example a cylinder, the width of the implant may be a limiting factor as to the height of the implant and therefore to the possible usefulness of the implant.
Implants having non-arcuate upper and lower opposed portions may be impacted into a space resembling the restored disc space and need only be placed against a “decorticated endplate.” A decorticated endplate is prepared by the surgeon to provide access to the underlying vascular bone. Some of the advantages provided by implants having non-arcuate opposed portions include: 1) preserving the best bone in the endplate region; 2) the height of the implant is independent of its width; 3) the implant can be of a geometric shape and the opposed upper and lower surfaces can be flat; 4) the implants can be installed as part of a modular unit; and 5) the implants can provide a broad surface contact. Some of the disadvantages provided by implants having non-arcuate opposed portions include: 1) the implants cannot be threaded in and must be impacted into the installation space; and 2) the recipient site may be more difficult to prepare.
Human vertebral bodies have a hard outer shell of compacted dense cancellous bone (sometimes referred to as the cortex) and a relatively softer, inner mass of cancellous bone. Just below the cortex adjacent the disc is a region of bone referred to herein as the “subchondral zone”. The outer shell of compact bone (the boney endplate) adjacent to the spinal disc and the underlying subchondral zone are together herein referred to as the boney “end plate region” and, for the purposes of this application, is hereby so defined. A circumferential ring of dense bone extends around the perimeter of the endplate region and is the mature boney successor of the “apophyseal growth ring”. This circumferential ring is formed of very dense bone and for the purposes of this application will be referred to as the “apophyseal rim”. For the purposes of this application, the “apophyseal rim area” includes the apophyseal rim and additionally includes the dense bone immediately adjacent thereto. The spinal disc that normally resides between the adjacent vertebral bodies maintains the spacing between those vertebral bodies and, in a healthy spine, allows for the normal relative motion between the vertebral bodies.
FIG. 1 of the attached drawings shows a cross-sectional top plan view of a vertebral body V in the lumbar spine to illustrate the dense bone of the apophyseal rim AR present proximate the perimeter of the vertebral body V about the endplate region and an inner mass of cancellous bone CB. The structure of the vertebral body has been compared to a core of wet balsa wood encased in a laminate of white oak. The apophyseal rim AR is the best structural bone and is peripherally disposed in the endplate of the vertebral body.
FIG. 2 is a top plan view of a fourth level lumbar vertebral body V shown in relationship anteriorly with the aorta and vena cave (collectively referred to as the “great vessels” GV). FIG. 3 is a top plan view of a first sacral level vertebral body V shown in relationship anteriorly with the iliac arteries and veins referred to by the designation “IA-V”. Because of the location of these fragile blood vessels along the anterior aspects of the lumbar vertebrae, no hardware should protrude from between the vertebral bodies and into the great vessels GV and iliac arteries and veins IA-V.
Implants for use in human spinal surgery can be made of a variety of materials not naturally found in the human body. Such materials include surgical quality metals, ceramics, plastics and plastic composites, and other such materials suitable for the intended purpose. Further, these materials may be absorbable, bioactive such as an osteogenic material, or be adapted to deliver and/or contain fusion promoting substances such as any of bone morphogenetic protein, hydroxyapatite, and genes coding for the production of bone, and/or others. Fusion implants preferably have a structure designed to promote fusion of the adjacent vertebral bodies by allowing for the growth of bone through the implant from vertebral body to adjacent vertebral body. This type of implant is intended to remain indefinitely within the patient's spine unless made of a resorbable or bioresorbable material such as bone that can be biologically replaced in the body over time such that it need not be removed as it is replaced over time and will no longer be there. Implants may be sized to have a width generally as great as the nucleus portion of the disc or as wide as the area between the limit lines LL as shown in FIG. 4. There are at least two circumstances where the use of such a wide implant is not desirable. Under these circumstances, the use of a pair of implants each having a width less than one half the width of the disc space to be fused is preferred. The first circumstance is where the implants are for insertion into the lumbar spine from a posterior approach. Because of the presence of the dural sac within the spinal canal, the insertion of a full width implant in a neurologically intact patient could not be performed from a posterior approach. The second circumstance is where the implants are for endoscopic, such as laproscopic, insertion regardless of the approach as it is highly desirable to minimize the ultimate size cross-sectionally of the path of insertion.
The ability to achieve spinal fusion is inter alia directly related to the vascular surface area of contact over which the fusion can occur, the quality and the quantity of the fusion mass, and the stability of the construct. The overall size of interbody spinal fusion implants is limited, however, by the shape of the implants relative to the natural anatomy of the human spine. For example, if such implants were to protrude from the spine they might cause injury to one or more of the proximate vital structures including the large blood vessels or neurological structures.
FIG. 4 shows a top plan view of the endplate region of a vertebral body V with the outline of a related art implant A and implant 100 of one embodiment of the present invention installed, one on each side of the centerline of the vertebral body V. The length and width of related art implant A is limited by its configuration and the vascular structures anteriorly (in this example) adjacent to the implantation space. The presence of limiting corners LC on the implant precludes the surgeon from utilizing an implant of this configuration having both the optimal width and length because the implant would markedly protrude from the spine.
Related art implants also fail to maximally sit over the best structural bone, which is located peripherally in the apophyseal rim of the vertebral body and is formed of the cortex and dense subchondral bone. The configurations of previous implants do not allow for maximizing both the vital surface area over which fusion could occur and the area available to bear the considerable loads present across the spine. Previous implant configurations do not allow for the full utilization of the apophyseal rim bone and the bone adjacent to it, located proximate the perimeter of the vertebral body to support the implants at their leading ends and to maximize the overall support area and area of contact for the implants. The full utilization of this dense peripheral bone would be ideal.
Therefore, there is a need for an interbody spinal fusion implant having opposed portions for placement toward adjacent vertebral bodies that is capable of fitting within the outer boundaries of the vertebral bodies between which the implant, is to be inserted and to maximize the surface area of contact of the implant and vertebral bone. The implant should achieve this purpose without interfering with the great vessels or neurological structures adjacent to the vertebrae into which the implant is to be implanted. There exists a further need for an implant that is adapted for placement more fully on the dense cortical bone proximate the perimeter of the vertebral bodies at the implant's leading end.